What is attempting an injection attack on a web server based on responses to True/False questions called?

DBMS-specific SQLi exploits syntax or functions unique to a particular database engine such as MySQL or MSSQL, which is unrelated to whether the attacker can observe direct output.

Compound SQLi refers to chaining multiple attack techniques together such as combining SQLi with other exploit types, not a method based on true/false inference.

Blind SQL injection is used when an application does not return query results directly in the HTTP response, forcing the attacker to ask the database true/false questions (Boolean-based blind) or trigger time delays (time-based blind) to extract data one bit at a time. The defining characteristic is inferring information from indirect behavioral responses rather than reading data directly from the page.

Classic (in-band) SQLi returns data directly within the application's HTTP response, which is the opposite of the indirect inference technique described in the question.